This invention relates to medical and dental X-ray systems and more particularly is concerned with circuits for affecting the voltage or current applied to the filament of an X-ray tube.
A simple and cost efficient X-ray system includes a high voltage transformer for applying high voltage in the order of 80 kilovolts across the cathode and plate of a Coolidge type X-ray tube. The primary of the high voltage transformer is energized by line voltage. The cathode of the tube is heated by a filament, which is supplied voltage by means of a low voltage transformer also operating from line voltage. In many X-ray tubes, the cathode and the filament are electrically connected and in fact may be the same structure. Often, for cost purposes, the output of the high voltage transformer is connected directly to the tube without any intervening rectifiers. The tube will then act as a self rectifier, conducting only during alternate half cycles of the high voltage. During conduction, electrons emitted from the hot cathode strike the plate of the tube which reacts by emitting X-rays. For a given time interval, the dosage of X-rays emitted is a function of tube voltage and tube current. To provide correct dosage, it is common to stabilize or control both or either tube voltage and current, so that variations in the line voltage will not seriously effect X-ray dosage. Quite a few schemes are known to stabilize current and voltage. Of these, many are rather costly. Furthermore, developments in high speed X-ray film have allowed dosage to be reduced to only a few half cycles, which may not be enough time for many systems to stabilize the current. It has therefore, become the practice to preheat the filament of the X-ray tube prior to the application of high voltage for an interval called preheat time.
It is known that the current of the tube is a function of filament temperature as well as other factors. The higher the filament temperature, the more current will flow from the plate to the cathode. The flow of current through the tube changes the voltage from the transformer due to the high resistance of the high voltage winding. The higher the current flow through the tube, the higher will be the voltage drop and the lower the high voltage across the tube. While simple preheat circuits do somewhat stabilize tube current, they do not offer fine correction for variations in the line voltage. If the preheat time is too short, the filament is relatively "cold" and the first few current pulses will be low causing a corresponding high voltage to be developed. Conversely, too long a time causes excessive initial current pulses and the kilovoltage is reduced. In the prior art, the preset preheat time is normally an integral number of half cycles of the input power, the precision of timing can be no better than one-half cycle of power. Thus, a set preheat time may be perfect for one particular line voltage, but as soon as the line voltage changes, the preheat time becomes less than perfect and considerable errors occur.
It is the main object of this invention to overcome this limitation so as to provide filament preheat cycles having continuous compensation for line voltage variation.